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FACTA UNIVERSITATIS Series: Electronics and Energetics Vol. 26, No 3, December 2013, pp. 227 - 238 DOI: 10.2298/FUEE1303227W

MULTIPLE PURPOSE SPIN TRANSFER TORQUE

OPERATED DEVICES

Thomas Windbacher, Hiwa Mahmoudi, Alexander Makarov,

Viktor Sverdlov, Siegfried Selberherr

Institute for Microelectronics, TU Wien, Gußhausstraße 27-29/E360, 1040 Wien

Abstract. We summarize our recent work on a non-volatile logic building block required

for energy-efficient information processing systems. A sequential logic device, in

particular, an alternative non-volatile magnetic flip-flop has been introduced. Its

properties are investigated and its extension to a very dense shift register is demonstrated.

We show that the flip-flop structure inherently exhibits oscillations and discuss its spin

torque nano-oscillator properties.

Key words: spin transfer torque, non-volatile logic, flip-flop, latch, shift register, nano-

oscillator

1. INTRODUCTION

Due to the rising efforts to scale CMOS devices down with every technology node,

according to the ITRS [1], new technological processes incorporating global and local

strain, new materials for high-k/metal gates, and new fin-FET and three-dimensional tri-

gate device structures must be introduced. At the same time power consumption and in-

terconnect delay start to become an important performance bottleneck [2]. One possible

way to reduce the power consumption of CMOS circuits is to shutdown latent circuit

parts completely and to power them up only when they are needed. This transition from

normally "ON" to normally "OFF" circuits requires the successive reevaluation of all

computational building blocks. The use of non-volatile elements which do not need

power to keep their state is paramount for the success of this new type of information

processing. At this point the exploitation of spin as degree of freedom comes in handy,

due to its non-volatility, high endurance, and fast operation [3]. Furthermore, it allows

not only to store information efficiently, but also to introduce new approaches to how the

actual information is processed and the information transport is handled [4], [5]. Thus, it

also allows to look for alternatives to the von Neumann architecture, where successively

information is moved between the memory unit and the processing unit through a - now-

adays performance limiting - common bus.

Received December 12, 2013

Corresponding author: Thomas Windbacher Institute for Microelectronics, TU Wien, Gußhausstraße 27-29/E360, 1040 Wien, http://www.iue.tuwien.ac.at

(e-mail: [email protected])

http://www.iue.tuwien.ac.at/

228 T. WINDBACHER, H. MAHMOUDI, A. MAKAROV, V. SVERDLOV, S. SELBERHERR

The most obvious potential application of non-volatile spintronic devices as CMOS

alternative is dynamic and static random access memory. The discovery of the giant

magneto resistance (GMR) [6], [7] and the tunnel magneto resistance (TMR) [8] effects

further boosted interest in magnetic devices, which results in improving their perfor-

mance and bringing them to commercial applications [9], [10]. At its earlier stage this

research led to the development of magnetic memory arrays [9], [11] for random access

memory applications: MRAM. Initially, their logic states had to be written by a magnetic

field, requiring an extra wire for field generation. Unfortunately, this caused high writing

energies and unfavorable scaling behavior and thus was prohibitive for (very) large scale

integration (VLSI). This problem was resolved by first theoretically predicting and later

experimentally justifying the spin transfer torque effect [12], [13]. The spin transfer

torque (STT) effect allows a purely electric control of the magnetization orientation and

thus leaves the writing wire generating the magnetic field in MRAM superfluous. How-

ever, the current value needed to invert the magnetization of the free layer in magnetic

tunnel junction (MTJ) is still rather high, and its reduction is the main goal of research to

make this technology competitive with CMOS. Utilization of MTJs with perpendicular

magnetic anisotropy allows reducing the required writing power to a level, where it can

compete with CMOS SRAM cache [14], [15], [16], [17]. Several products based on spin

transfer torque magnetic random access memory (STT-MRAM) technology have been

recently introduced [18], [19], [20], [21].

However, besides the apparent MTJ applications for memory cells, new spintronic

devices open up additional opportunities for replacing CMOS based counterparts [3].

Below we introduce a non-volatile magnetic flip-flop as an information processing unit.

2. NON-VOLATILE MAGNETIC FLIP-FLOP

As already mentioned, the ever increasing demand in fast and cheap memory as well

as in electronics in general has driven the scaling efforts in CMOS since its very begin-

nings. Today, pushing the limits of integration density is still a major concern, but power

efficient computing gains more and more momentum. In addition to memory and combi-

natorial logic mostly employed in modern integrated circuits, sequential logic attracts

close attention. Sequential logic can be based on memory elements for information pro-

cessing. Its present output states not only depend on its present inputs, but also on its in-

put history [22]. Sequential logic devices are fundamental building blocks of digital elec-

tronics, and flip-flops as well as latches belong to this group of logic. The transition from

normally "On" to normally "Off" logic requires a redesign of these devices as well.

Even though already several circuit designs have been proposed to meet this goal,

e.g., Zhao et al. [23], these circuits are commonly CMOS-MTJ hybrids. The non-volatil-

ity is introduced by the MTJs, however, the MTJs act merely as ancillary devices by only

holding the information without any further functionality. The actual logic operations are

carried out in the CMOS elements. The introduction of non-volatility through MTJs adds

complexity because of the need for converting the information between resistance states

of the MTJs and voltage or current signals required for CMOS operation every time data

is written, read, or transferred.

Multiple Purpose Spin Transfer Torque Operated Device 229

Therefore, we propose a novel non-volatile magnetic flip-flop which shifts the actual

logic operation and thus the information processing from the electric signal CMOS

domain to the magnetic domain [24]. This allows creating denser and simpler layouts as

well as harvesting the beneficial features related to spintronics.

Fig. 1 Proposed non-volatile magnetic flip-flop

2.1. Structure and working principle

The non-volatile magnetic flip-flop consists of three fixed anti-ferromagnetically cou-

pled polarizer stacks (see Fig. 1). Two polarizer stacks are for input (A, B) and one po-

larizer stack is for readout (Q). They exhibit a fixed perpendicular magnetization orienta-

tion (parallel to the z-axis) and due to their anti-ferromagnetic configuration negligible

stray fields are assumed. The polarizer stacks are positioned on top of tunnel barriers (e.g.

Cu, Al2O3, and MgO) and share a common free layer. The common free layer exhibits a

perpendicular anisotropy with a constant uni-axial anisotropy K1 (parallel to the z-axis). It

is further assumed that the ratio of the common free layer is 4 to 1 with the size of the

short side of the layer ranging from 10 nm to 30 nm. The long side of the free layer is

along the y-axis, while the short side lies parallel to the x-axis.

Furthermore, a positive electric current is defined as flowing from the contacts (A, B,

Q) through the free layer (negative z-direction), while at the same time electrons flow in

the opposite direction (positive z-direction).The logic information is stored via the mag-

netic orientation of the free layer, and the polarity of the input pulses is mapped to logic

"0" and "1", respectively.

A current pulse applied to one of the input stacks (A or B) exerts a spin transfer

torque on the magnetization in the corresponding free layer region of the input stack and

generates a precessional magnetization motion. This precessional motion couples through

the magnetic exchange to the rest of the free layer and creates a spin wave which travels

through the common free layer heading to its opposite end, where it is reflected and

moves back, gets reflected, and pushed again and so on [25]. During this kind of oscillat-

ing spin wave motion, the localized magnetic moments in the common free layer are ex-

cited and their precessional motions build up, until the magnetization eventually passes

the energy barrier between the two stable states and relaxes into the other stable state fast.

If now, instead of one, two synchronous current pulses are applied to the input stacks A

and B, two moving spin waves are generated. Depending on their polarity they either try

to enforce the switching of the free layer magnetization or hold it in its current position.

There are four possible input combinations. In the case of two positive currents both

generated torques align the magnetization parallel to the z-axis. Therefore, the magneti-

zation orientation stays unchanged. If both input currents are negative, both generated


torques try to flip the magnetization orientation. In this case both spin waves add up

leading to a faster switching compared to a single input current. Finally, in the case of

opposing input current polarities one torque always tries to hold the current magnetiza-

tion state, while the other strives to flip it. Since these two contributions compensate each

other, the magnetization direction of the free layer remains the same.

Thus, two sufficiently high and long pulses with identical polarity either write logic

"0" or "1" into the common free layer, while two pulses with opposing polarities cancel

each other and the initial magnetization state is held representing a sequential logic func-

tion needed for flip-flop logic. If one of the inputs is inverted, the resulting logic sets or

resets the held state, when the input signals are opposing, and holds its state for identical

input polarities. This corresponds to RS flip-flop logic, but without forbidden input com-

binations as compare to its transistor counterpart [22].

2.2. Device characteristics

In order to test our proposed device a series of micromagnetic simulations was carried

out [26], [27]. Three different free layer sizes with length to width ratio of four to one

were studied. It was further assumed that the common free layer featured layer widths of

10nm, 20nm, and 30nm, a layer thickness of 3nm, a perpendicular uni-axial anisotropy

K1=105 J/m

3 along the z-axis, and that the spin barrier connecting the polarizer stacks to

the free layer is made out of copper (cf. Fig. 1). The saturation magnetization of the

common free layer was considered as MS=4×105A/m, an exchange constant of

Aex=2×10-11

J/m, and a spin current polarization P of 0.3. As mentioned in Section 2.1, the

polarizer stacks consist of anti-ferromagnetically coupled layers. Therefore, stray fields

are neglected.

The magnetization dynamics is described by the Landau-Lifshitz-Gilbert equation

[28], [29]:

T

dt

mdmHm

dt

mdeff

, (1)

where ⃗⃗ denotes the reduced magnetization, γ=2.211×105m/As the Gilbert gyromagnetic

ratio, α = 0.01 the dimensionless damping constant, ⃗⃗ the effective field in A/m. The

last term in (1) describes the spin transfer torque ⃗ with the following spin transfer torque

model [30]:

)´()1()1( 22

2

0

pmmpmpm

P

Ml

J

eT

S

. (2)

denotes the Planck constant, µ0 the permittivity of vacuum, e the electron charge, J the

applied current density, l the free layer thickness, MS the magnetization saturation, P the

polarization, the unit polarization direction of the polarized current, and Λ=2 a fitting

parameter handling non-idealities. Furthermore, ⃗⃗ is calculated from the functional

derivative of the free energy density functional containing uni-axial anisotropy, ex-

change, and demagnetization contributions [31].


Fig. 2 Switching times for identical input pulses as a function of applied current density

Fig. 3 Switching times for opposing input pulses as a function of applied current density. m

denotes a negative polarity current pulse, while p denotes a positive polarity current

pulse. For instance pmp (A, B, free layer) means a positive current pulse applied at

contact A, a negative current pulse applied at B, and a magnetization orientation of

the free layer along z-axis

The data points in Fig. 2 and Fig. 3 are averaged over 100 samples. The error bars de-

pict an error range of ±3σ. Fig. 2 shows the switching times for the set and reset operation

as a function of applied current densities and different layer sizes. The corresponding

switching times are defined as the time it takes from the onset of the current pulse until

80% of the final magnetization state is reached. One can see that at low current densities

it takes relatively long to build up the required precessional motion for flipping the layer

magnetization, while with increasing current densities the switching times decreases fast.

The difference in the threshold current densities for different geometries can be explained

as follows. Reducing the layer width reduces the demagnetization factor along z but at

the same time the demagnetization factors along x and y start to increase [32]. This leads

to a smaller depolarization field along z and a bigger net barrier parallel to the z-axis,

since the depolarization field opposes the anisotropy field. Therefore, the smaller the

layer width the higher is the switching barrier and the required current to pass the barrier.

For a pulse length of 20ns switching starts at 4×1010

A/m2 (16±3.5ns), 5×10

10A/m

2


(13±3ns), and 6×1010

A/m2 (17.5±4.3ns) for 30nm, 20nm, and 10nm width, respectively. It is

worth to mention that, the switching speed becomes almost size independent around

≈1011

A/m2 until ≈5×10

12A/m

2 and the found switching times are in the order of ≈20ps to

≈100ps.

In contrast to the set and reset operation, the hold operation must not lead to a change

in the magnetization state. This is the case at opposing input pulses (see Fig. 3). For a

reliable flip-flop operation the magnetization ⃗⃗ must relax back to its initial orientation,

when the currents are turned off. Starting with an initial magnetization state along the

positive z-axis (as shown in Fig. 1), the total average magnetization projection should not

become negative during its precessional motion. If the magnetization overcomes the state

separating energy barrier and its projection on the z-axis becomes negative ( ), the

final magnetization state starts to depend on the pulse length. In other words, this is the

point, when the held information is lost.

The devices behave as required for current densities below 2×1012

A/m2 for 10nm

width, 7×1011

A/m2 for 20nm width, and 5×10

11A/m

2 for 30nm width (cf. Fig. 3). Close to

the respective current densities the spin wave generated at A (B) is not sufficiently

damped anymore and oscillatory motions start to build up. In fact, above the shown limits

the oscillations can become so strong that they cross the xy-plane and the initial

magnetization orientation is lost. Due to the geometrical confinement the oscillation

frequencies of the excited spin waves as well as the onset current densities for their

excitation depend on for the shared free layer sizes.

Already at this point the proposed non-volatile magnetic flip-flop is very attractive. In

addition, it requires less space than its CMOS counterparts, e.g., eight or twelve

transistors for a classical simple or clocked RS flip-flop, correspondingly [22], and it is

also more space efficient than hybrid MTJ/CMOS circuits, for example, the flip-flop

proposed in [23], which requires seven transistors and two magnetic tunnel junction

memory elements.

Instead the non-volatile magnetic flip-flop needs only three magnetic stacks (spin

valve and/or MTJ) sharing a common free layer. Even more, there are no forbidden input

combinations ( ̅ ) making the circuit and logic design easier. Due

to its back end of line compatibility and stack friendly topology (see Fig. 4), it is also

attractive for large scale integration.

2.3. Shift-register

In [33] we proposed an application of the flip-flop for a non-volatile shift register.

The shift register features an extremely dense layout and allows further reduction of the

required space (cf. Fig. 4).

The novel shift register topology takes advantage of the peculiarities of the non-

volatile flip-flop. Arranging two rows of flip-flops in two different levels along a line (as

shown in Fig. 3) or in a cross-like structure [33] and applying two time shifted clock

signals (Clk1 and Clk2, see Fig. 5 and Fig. 6), the information contained in one flip-flop

can be successively passed to the next subsequent flip-flop via the STT effect.

The shift register works as follows. Each flip-flop is alternately operated in a writing

and a reading mode. In the writing mode the inputs Ai and Bi are active, while Qi is

inactive. In this mode one input is fed with the information/signal while the other is fed

by one of the two clock signals (depending on its level, cf. Fig. 5). In the reading mode Ai


and Bi are inactive and Qi is active. A save decoupling between the two operation modes

is achieved by a time shift between the two clock signals (see Fig. 6).

In order to avoid the unintentional flipping of the common free layer (also known as

read disturbance), the pulse time mismatch between the reading and writing mode is

used. While in reading mode only the part of the common free layer under the Qi region

experiences a spin torque, in writing mode two regions Ai and Bi exert a spin torque on

the common free layer. Therefore, it takes about twice as long to change the magnetiza-

tion orientation in reading mode than in writing mode provided the currents are the same.

Let us illustrate the working principal with an example (see Fig. 4, Fig. 5, and Fig. 6).

Applying an input pulse to A1 and a first clock signal Clk1 to B1 to the first flip-flop

(Fig. 4, left) will set or reset the flip-flop, if both pulses exhibit the same polarity. When

the input signal and the clock signal Clk1 exhibit the same torque on the shared free layer,

information is written into the first free layer and can be accessed via Q1. At the next step

clock signal Clk1 turns inactive and Clk2 becomes active (cf. Fig. 6). This time the first

flip-flop is in reading mode and a current passes through the first free layer and the spin

barrier separating the two neighboring free layers before it enters to the second free layer

and exerts a spin torque on the second free layer. At the same time the clock signal Clk2

creates spin torque in the B2 region and the interaction between the two spin torque

sources either aids or suppresses the switching of its magnetization state. Afterwards the

clock signal Clk2 is switched off again and the clock signal Clk1 is turned on. Hence, the

second flip-flop is now in reading mode and information is passed via the STT effect to

the third free layer and so on. It is necessary to mention that the voltage or current signals

passing the information at Qi must be synchronized with respect to the corresponding

clock signals and that due to the additional degree of freedom introduced by the variable

shared free layer magnetization, Qi must be fed with one type of polarity only. This is

necessary to compensate the changing free layer magnetization in region Qi. The clock

signals Clk1 and Clk2 require a positive and negative pulse within their signal period to

guarantee that the information stored in one shared free layer will be successfully passed

into the next subsequent flip-flop, since only by two identical input pulses the flip-flop is

written.

Fig. 4 Proposed non-volatile magnetic shift register

Fig. 5 Circuit diagram of non-volatile magnetic shift register


Fig. 6 Signal timing example for the non-volatile magnetic shift register showing

the successive propagation of the input signal through register

2.4. Intrinsic oscillator

Despite its use as a sequential logic building block the flip-flop structure also features

large and stable oscillations (see Fig. 3 and Fig. 7). This is bad for logic applications, but

quite appealing for exploitation as a nano-scale oscillator. Oscillators constitute a very

basic building block and are ubiquitous in modern electronics. They are needed in meas-

urement, navigation, communication systems, and more. Their periodic output signals are

used manifold, for example, for clocking digital circuits, generating electromagnetic

waves, in high speed digital systems, and many more. Spin torque nano-scale oscillators

are so attractive because of their cost effective on-chip integration as microwave oscilla-

tors, their nano-scale size, frequency tunability, broad temperature operation range, and

CMOS technology compatibility [34], [35], [36], [37], [38]. Even though substantial pro-

gress has been achieved and they can now operate without an external magnetic field,

their still rather low output power limits their practical implementation [39], [40].

As discussed in Section 2.2, we observe oscillations in the common free layer, if cur-

rent densities with opposing polarities above the respective thresholds are applied (shown

in Fig. 3) [33], [41]. The excited precessional motions form large and stable orbits and do

not require an external magnetic field (see Fig. 7).

Fig. 7 Plot of the averaged and normalized magnetization

of the common free layer for 10nm, 20nm, and 30nm, respectively


Fig. 8 Frequency as a function of applied current density and free layer size

The occurring precessional motions are a superposition of two movements - an in-

plane precession (fx,z) and an out-of-plane oscillation (fz). Plotting the found oscillation

frequencies as a function of applied current density and free layer size shows that for a

fixed current density the frequency fz for the out-of-plane component oscillations (filled

symbols) is two times the corresponding in-plane precession fx,y frequency (open sym-

bols). It also can be observed that for different shared free layer sizes the precessional

frequencies change (see Fig. 8). For the 30nm×120nm structure the stable precessional

motion starts at ≈250 MHz (≈5×1011

A/m2) and increases up to ≈450 MHz (1×10

12A/m

2),

while for 20nm×80nm the precessions start at ≈1 GHz (1.×1012

A/m2) and increases to

≈ 1.17 GHz (1.25×1012

A/m2). For the 10nm×40nm structure they range from ≈3.22 GHz

(1.6×1012

A/m2) to ≈ 3 GHz (2.05×10

12A/m

2).

Further analysis shows that in the case of a single pulse at one input polarizer the

magnetization of the free layer is inverted, even though the generated spin torque acts

only on the free layer below the corresponding polarizer stack. Therefore, in this setup

two input pulses are required for the generation of oscillation. This hints already to the

explanation how the oscillations are excited. For example, if a positive current pulse is

applied through region A (shown in Fig. 1), the generated polarized current will exert a

spin torque on the local magnetic momenta and excite local precessional motions, which

try to flip the free layer magnetization. Through the exchange coupling these precessional

motions start to move out of the region where they are generated and travel through the

whole free layer (also known as spin wave). When the spin wave reaches the opposite

end of the free layer (B), it experiences a second torque which tries to preserve the mag-

netization orientation of the free layer (negative current). The wave gets reflected, heads

back to region A where it is pushed out again, and so on. The result is a stable preces-

sional motion in the whole free layer. This explanation is consistent with the time and

position resolved magnetization simulations we conducted and also explains the depend-

ence of the frequency on the free layer size [38]. While other groups reported a localiza-

tion of the oscillations below the spin injection site, in our device the full layer contrib-

utes to the precessions [36], [37], and [38]. We think this difference stems from the rather

strong stray field in the setups used in [36], [37], and [38], while in our case the stray

field is negligible.

However, in order to extract power from the oscillator the projection of the preces-

sional motion on the pinned layer magnetization orientation should be as big as possible.


Unfortunately, the perpendicular polarizers (see Fig. 1) lead to an extraction of the much

weaker vertical oscillations fz (see Fig. 7). A possible way to circumvent this obstacle is

to change the orientation of the output polarizer Q to an in-plane configuration and ex-

tract the signal from there. Thus, the much bigger component can be used. But

this also comes at a price; since the power extracted from Q is proportional to the power

put through Q, it must be sufficiently small to not disturb the oscillations.

Therefore, one might instead of changing Q to an in-plane orientation, modify A and

B to in-plane configurations [39]. This case was tested in [41], where it was shown that

the free layer magnetization also preforms large in-plane precessions. The observed sta-

ble oscillations exist even when the current is driven through a single stack. The current

through the second stack allows tuning the frequency. If the current through the second

stack exhibits the opposite polarity, the frequency of the oscillations is almost doubled.

Because of the in-plane orientations of the polarizer stacks, the large component

is extracted and the available microwave power is increased. Because the whole free

layer exhibits the oscillations when the excitation sources are at its opposite ends, wire-

less power extraction and further distribution is an interesting option for this structure.

3. OUTLOOK AND CONCLUSION

Even though great progress has been made over the last years, there are still chal-

lenges to be resolved to bring spintronics into mainstream electronics. One reason is the

maturity and excellence of CMOS technology itself. While the switching power of STT

based memory has become comparable with that of CMOS based SRAM and DRAM

[10], [14], [15], [16], and [17] its integration density is currently not as good as in state of

the art CMOS solutions. Spintronics for logic applications is under extensive develop-

ment. One of the show stoppers is the rather high energy required for switching: switches

based on the STT effect require still about 100 times more switching power than a CMOS

transistor [3]. Thus, the real challenge to bring the spin-driven devices close to commer-

cial applications is to reduce the required switching power for memory and logic. For

STT based nano-oscillators an increase of the output power is a pressing issue. A prom-

ising path to reach this goal is to migrate from current controlled switching to voltage

controlled switching. There are many promising results related to the voltage control of

magnetic properties. For instance, Wang et al. [42] have shown electric-field-assisted

reversible switching in CoFeB/MgO/CoFeB magnetic tunnel junctions with interfacial

perpendicular magnetic anisotropy, where the coercivity, the magnetic configuration and

the tunneling magnetoresistance can be manipulated by voltage pulses associated with

much smaller current densities (≈1.2×106A/m

2). Despite the critical switching power re-

duction also other essential features like fast switching in the below nanosecond regime

[43] and voltage control of the exchange bias at room temperature have been demon-

strated [44]. It also has been shown by Nozaki et al. [45] that electric-field-induced ferro-

magnetic resonance excitation by means of voltage control over the magnetic anisotropy

in a few monolayers of FeCo at room temperature is feasible. So we conclude that there

is still plenty of space left to explore and to further develop spintronics and its application

in large scale applications.

Acknowledgement: This work is supported by the European Research Council through the grant

#247056 MOSILSPIN.


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